Frequency regulating device for A.C. systems interconnected through D.C. systems

ABSTRACT

In a system interconnection composed of two a.c. systems, two converters connected with the respective a.c. systems and a d.c. power transmission system, one of the converters or rectifier is subject to constant current control and the other or inverter is subject to constant margin angle control. In the control system, the difference between the frequencies of the a.c. systems to which the respective converters are connected is utilized for modifying the set point of control. The greater the frequency difference, the rate of modification of the set point is increased so as to minimize the frequency difference between the a.c. systems remaining after control.

United States Patent [191 Watanabe et a].

[ Sept. 16, 1975 [75] Inventors: Atsumi Watanabe; Hisao Amano; Akira Isono, all of Hitachi, Japan [73] Assignee: Hitachi, Ltd., Japan [22] Filed: Mar. 11, 1974 [21] Appl. No.: 450,187

3,470,442 9/[969 Ainsworth 32l/2 3,526,779 9/1970 Uhlmann 307/20 3,644,820 2/l972 Ainsworth 321/2 Primary ExaminerWilliam H. Beha, Jr. Attorney, Agent, or FirmCraig & Antonelli 5 7 ABSTRACT In a system interconnection composed of two a.c. systems, two converters connected with the respective a.c. systems and a dc. power transmission system, one of the converters or rectifier is subject to constant current control and the other or inverter is subject to constant margin angle control. In the control system, the difference between the frequencies of the a.c. systems to which the respective converters are connected is utilized for modifying the set point of control. The greater the frequency difference, the rate of modification of the set point is increased so as to minimize the frequency difference between the a.c. systems remaining after control.

7 -,Pmmm$111M RTE J PTB T .y W T A FREQUENCY R2 FREQUENCY DETECTOR DETECTOR F VA I F V B) FIG. 3

Fo+ AF APT AF PATENTEIBSEPUIIS ms Q VOLTAGE F G;. 6 v

OUTPUT OF AA? OUTPUT OF AA8 P OUTPUT 0F AA4 "OUTPUT OF AAS v PATENTEU SEP 1 6 ms sum 5 o FIG. 9

S IOO F FREQUENCY REGULATING DEVICE FOR A.C. SYSTEMS INTERCONNECTED THROUGH D.C. I

SYSTEMS This invention relates to the art of system interconnection, and more particularly to a control system for a system interconnection in which two a.c. systems are interconnected by a dc system so that power can be smoothly and reliably transmitted from one of the a.c. systemsto the other and the frequency in the individual a.c. systems can be fully stabilized.

It is commonly known that parallel operation of power systems in a large-scale electrical power system provides such great economical advantages that a power plant of large capacity can be constructed to serve the entire systems and the power requirement for stand-by service can be reduced. On the other hand, however, this manner of operation involves various defects relating to the reliability of power supply in that circuit breakers of large capacity are required to deal with an unusually large current appearing as a result of occurrence of short-circuit or other fault in the system and that such fault may lead to large-scale interruption of service in the entire systems.

In view of teh defects above described, an attempt has been made in which an electrical power system is divided into a plurality of systems of small scale and these subdivided systems are interconnected so that these subdivided systems can beoperated substantially as a single system and an unusually large current may not appear even in the event of'occurrence of fault. This interconnection is broadly divided into a' so-called a.c. interconnection in which two a.c. systems are directly interconnected as disclosed in, for example, US. Pat. No. 3,657,728, and a so-called d.c. interconnection in which two a.c. systems are interconnected by a dc system. The present invention relates to the latter kind of interconnection.

In the latter kind of interconnection, a converter is disposed at each of the interconnected points between two a.c. systems and these two converters are connected to each other'by a dc. transmission line. One of these two converters is a rectifier for converting a.c. power into d.c.power, and the other converter is an im ve'rter for converting the dc. power into a.c. power again. These two converters are controlled by detecting the difference between the frequencies of the a.c. systems to which they are connected. In other words, these two converters are controlled so that power can be transmitted from the a.c. system of higher frequency to the a.c. system of lower frequency depending on the frequency difference. When the converters are controlled in the manner above described, the generated power and load in these two a.c. systems can be balanced and the interconnecting means can 'attain the desired function. Thus, even when fault may occur'in one of the a.c. systems, an unusually large current due to this fault is not supplied to the other a.c. system due to the fact thatthe latter is separated from the former by the dc system. i i

The dc interconnection is useful for the interconnection between two a.c. systems as above described. However, power transmission between the two a.c. systems while merely controlling the converters depending on the frequency difference between these two a.c. system is insufficient in that an offset is inevitably involved in the control system. Thus, the prior art manner of interconnection has been defective in that the desired interconnecting function cannot be fully attained since frequency variations of considerable degree result in a great frequency difference remaining still after control.

It is therefore an object of the present invention to provide a control system fora system interconnection consisting of a.c. systems interconnected by a dc. system, in-which means are provided so that any appreciable frequency difference may not remain after control even when considerable frequency variations may occur in the a.c. systems interconnected by the dc. system.

The present invention is featured by the fact that power being transmitted from one of the a.c. systems to the other is controlled in non-linear fashion depending on the difference between the frequencies of the two a.c. systems so as to minimize the frequency difference remaining after control.

The above and other objects, features and advan-. tages of the present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a simplified block diagram of a prior art system interconnection for illustrating the necessity of the present invention;

FIG. 2 is a graph showing the characteristic of the control means shown in FIG. 1;

FIG. 3 is a block diagram showing the interconnection in FIG. 1 in terms of transfer functions;

, FIG. 4 is a block diagram of an embodiment of the present invention;

FIG. 5 is a graph'showing the manner of operation of the two converters shown .in FIG. 4;

FIG. 6 is a graph showing the operating characteris-v tics of parts of the control system shown in FIG. 4;

FIGS. 7, 9 and II are block diagrams of parts of other embodiments of the control system according to the present invention;

FIGS. 8, l0 and 12 are graphs showing the operating characteristics of the'embodiments shown in FIGS. 7, 9 and 11 respectively.

1 Referring to FIG. 1 showing a prior art system interconnection two a.c. systems A and-B are interconnected by an converter C a dc. transmission line L and another converter C Frequency detectors FVA and 'FVB detect the frequencies of the a.c. systems A and B on the basis-ofthe voltagesapplied from voltage transformers PTA and PTB respectively and deliver positive d.c. voltages-of levels proportional to the respective detected frequencies. A frequency control circuit AF is provided'for suppressing frequency variations, in the a.c. systems A and B. An adder AD in the frequency control circuit AF detects the difference between the output of the detector FVA and the output of the detector FVB, and the output of the adder AD is amplified by an operational amplifier AM so as to suitably modify the settings of the converters C and C gain of this amplifier AM is determined by the ratio between resistors R and R Therefore, AF and AP have aproportional relationship as shown in FIG. 2 in which AF is the frequency difference between the a.c. systems A and B detected by the adder AD and AP is the variation of the power being transmitted through the dc system'due to the variation of the output of the frequency control circuit AF relative to AF. In FIG. 2, it i's'su'pposed'that the direction of power transmission isfrom the system A to the system B and the frequency difference AF is positive when the frequency of the system A is higher than that of the system B.

Consider now the effect of suppression of frequency variations with such a manner of control. Suppose that afrequency difference of AF exists between the two a.c. systems A and B, the power being transmitted through the d.c. transmission line L is varied by AP-,- as a result of corresponding control, and consequently, the frequency difference is improved to AF. These val ues have a relationship as shown in FIG. 3. In FIG. 3, AF is the corrected frequency difference, G, is a transfer function given by dividing the power variation in the transmission line by the frequency difference, and G is a transfer function given by the respective systems A and B.

From FIG. 3, the following equation can be obtained:

Therefore,

AF/AF,, l/(l G,-G

The equation (2) exhibits the frequency improving effect by the d.c. interconnection shown in FIG. 1. Thus, the frequency variation can be suppressed to a fraction of the value obtained in the case where such d.c. interconnection is not provided, although the effect'of suppression differs depending on the values of G and G However, as will be apparent from FIG. 3., an offset is inevitably involved in the control system shown in FIG. I due to the fact that the control system is composed essentially of proportional elements only. Therefore, when the original frequency variation AF is con siderably large. the resultant frequency difference AF is correspondingly large and the desired control effect cannot be fully attained.

The present invention is based on the finding that an increase in G, in the equation (2) can reduce the frequency difference remaining after control, and instead of simply determining the power in the d.c. system so as to be proportional to the frequency difference, the rate of increase in the supplied d.c. power is increased with the increase in the frequency difference so as to increase G, in the equation (2) thereby minimizing the frequency difference remaining after control.

FIG. 4 is a block diagram of an embodiment of the present invention and like reference numerals are used therein to denote like parts appearing in FIG. 1. Referring to FIG. 4, the control system includes transformers TA and TB, d.c. reactors DCLA and DCLB, current transformers CTAl, CTAZ, CT B1 and CTBZ, and direct current transformers DCCTA and DCCT B. Automatic pulse phase shifters APPSA and APPSB are controlled by respective signals applied from constant current control means CIA and CIB or constant margin anglecontrol means 8A and 88 as described later. The constant margin angle control means 6A and 8B detect the margin angle of converters C, and C on the basis of the primary voltage and secondary current of the transformers TA and TB respectively and compare the detected values with a predetermined setting 5,, applied to terminals tAl and tBl to deliver a' voltage required for attaining the coincidence therebetween. The constant current control means CIA and CIB deliver a voltage required for attaining the coincidence between the current in transmission lines L and a predetermined setting. The automatic pulse phase shifters APPSA and APPSB select one of the outputs of the constant margin angle control means and constant current control means so as to apply a firing phase of suitable timing to the converters C, and C and may be any suitable ones known in the art. The present invention can be applied to a commonly known manner of control in which the rectifier is subject to constant current control and the inverter is subject to constant margin angle control, and such a case will be described with reference to FIG. 4. The constant margin angle control means and constant current control means employed in the present invention for the transmission power control may have a known structure and any especial description as to the structure of such means will not be given herein.

Power detectors PDA and PDB deliver a positive voltage whose level is proportional to the power being transmitted. Direct current detectors IDA and IDB deliver a voltage whose level is proportional to the direct current. These detectors may be any suitable ones known in the art and include therein proper means for smoothing out ripple components in the output. Various settings are applied to terminals tAl, tAZ, tA3 and tBl. Adders AD, ADAl, ADA2, ADBI, ADB2, subtract the input applied by the line having the symbol *minus from the input applied by the line having the symbol plus Thcre are a plurality of operational amplifiers AAl, AAZ, A81 and ABZ, a plurality of resistors RA], R A2, RBI, RBZ, associated with these operational amplifiers, and a plurality of diodes D,, D A switch SW is manually changed over to connect the terminal tA3 to one of contacts a and [2. Delay elements TDA and TDB are connected between the operational amplifier AAl and the adder ADA3 and between the operational amplifier ABl and the adder ADB3 respectively.

Before describing modification of the power being transmitted through the d.c. system on the basis of the frequency difference between the two a.e. systems A and B which is the object of the present invention, description will be given with reference to a common control in which the rectifier C, and inverter C are operated under constant current control and constant margin angle control. The power detectors PDA and PDB are constructed so that they deliver a d.c. voltage output of, for example, 10 volts when the rectifier C, and inverter C 2 are operating to deliver the rated output of, for example, 300 MW and they deliver zero volts when the output of the rectifier and inverter is zero. A voltage -P,, corresponding to the setting of transmitted power is applied to the terminal tA2. This voltage P,, is a dc. voltage of, for example, 10 volts when the rectifier and inverter are operating to deliver the rated output.- The current detectors IDA and IDB are constructed so that, when the rectifier C, and inverter C are operating with the rated output of, for example. 300 MW and the rated voltage of, for example, I25 kV and a current value of, for example, I200 A is detected, theydelivcr a d.c. voltage of, for example, 10 volts,

while when zero current is detected,.they deliverzero volt. A voltage Al corresponding to a current margin is applied to the terminal tA3 so that either the converters C or C can operate as a inverter. This margin is commonly selected to be of the order of percent of the rated current value, and therefore, a dc. voltage of 1 volt is commonly applied tothe terminal tA3. T he symbols and affixed to'various parts mean that the signals appearing at such parts are positive and negative relative to each other. i

Suppose that power is transmitted from the ac. system A to the ac. system B and the control system is operating to control the transmission of predetermined power between these two a.c. systems A andB as described with reference to FIG. 1. Supposefurther'that the output AP of a frequency control circuit AF described later is zero. Since AP 0, the voltage P,, applied to the terminal tA2 appears at the output of the adders ADAl and ADBl. These outputs are applied to the respective adders ADA2 and ADB2, and at the same time, to the respective adders ADA3and ADB3. The outputs of the power detectors PDA and PDB are also applied to the respective adders ADA2 and ADB2.

Since the system is operating under the rated condition, the outputs of the adders ADA2 and ADB2 are zero, and the outputs of the operational amplifiers AAl, A81 and delay elements TDA, TDB are also zero. Due to the fact that the power is being transmitted from the system A to the system B, the converter C and Cl; operate as a rectifier and an inverter respectively. The switch SW is connected to the contact 17 and the voltage Al applied to the terminal tA3 is applied to the adder ADB3. Therefore, the voltage P appears at the output of the adder ADA3 to be applied to the adder ADA4, and the voltage P AI appears at the output of the adder ADB3 to be applied to the adder ADB4 to provide the set points for the respective constant current control means CIA and CIB, As described previously, the outputs of the individual detectors and the settings applied to the individual terminals are selected to have corresponding dc voltage levels in the rated condition. Thus, any especial problem does not arise even when the power setting is selected to be the set point of the constant current control means. The outputs of the direct current detectors IDA and [D8 are applied in the illustrated polarity to the respective adders ADA4 and ADB4. Thus, the output of the adder ADA4 is approximately zero,'while the output of the adder ADB4 is Al, and the output of the adder ADB4 is greater than that of the adder ADA4 by AI. It is commonly known that, when the current margin AI is applied to only one of the two constant current control means and the outputs of the constant margin angle control means and constant current control means are selected by the automatic pulse phase shifters APPSA and APPSB for controlling the converters C and C in the manner above described, one of the converters to which Al is applied acts as an inverter and the other acts as a rectifier. Therefore, in the case which is presently described, rectifier C is controlled by the constant current control means CIA, while the inverter C is controlled by theconstant margin angle control means 813. These converters have operating characteristics.as shown in FIG. 5 in which the vertical axisrepresentslthe dlc. transmission line voltage and the horizontala tis replre sents the current. These converters are Operatedwith the voltage and current values shown by the dotted son,- the output of the power detector PDA is reduced and theoutput of the adderADAZ is changed to a positive value. The output of the adder ADA2 is amplified by the operationalamplifier AAl with the gain determined by the resistors RAl and RA2 to be applied to the. delay,el ement TDA. Therefore,.a reduction in the transmitted power causes an increase in the output of the adder ADA3 after a delay by the delay element TDA so as to increase the set point of the constant current control means CIA. As a result, the phase of the output of the automatic pulse phase shifter APPSA is variedto increase thetransmitted power. Conversely, when the transmitted power is increased, the output of the, adder ADA3 is reduced to reduce the transmitted power. The increase or decrease in the output of the adder ADA3 is determined by the gain of the opera tional amplifier AAZ which is determined by the resistors RA3 RA4. A reduction in the transmitted power is also detected by the power detector PDB re sulting in a variation of the output of the adder ADB3. However, this exerts no influence due to the fact that the inverter C. is operating under control of the constant margin angle control means.

The transmitted power can thus be suitably modified when a change occurs in the power being transmitted. From another standpoint, this means that the transmitted power can be controlled by apparently changing the power setting. The frequency control circuit AF described with reference to FIG. I is provided for this purpose, arid the output AP of this frequency control circuit AF is applied to the adders ADAI and ADBl as shown in FIG. 4. Thus, even when any direct change 'may not be observed in the transmitted power, occurrence of a frequency difference AF between the two systems A and B is apparently equivalent to a change of the power setting, and the power being transmitted can be controlled correspondingly. For example, when AP is positive, the tramsmitted power is increased, while when AP is negative, the transmitted power is decreased. However, as pointed out previously, the residual error after control is great when AP is merely proportional to AF. The operation'of the' frequency control circuit AF which is one of the features of the present invention-will now be described in detail.

The outputs of the frequency detectors FVA and FVB are applied by respective leads and 200 to the frequency control circuit AF and the output AP appearing from the frequency control circuit AF is applied to the adders ADAl and ADBl by a lead 300. As described previously, AF is positive when the frequency of the system A is higher than that of the system B. The relation between AF and AP is selected so that AP is also positivewhen AF is positive and the output voltage of the adder AD detecting AF has a negative polarity when AF is positive.

Referring to FIG. 4, the operational amplifiers AA3 to AA8 amplify the input AF with the gain determined by the ratio between the associated input resistor and the feedback resistor. A positive bias voltage is applied to the operational amplifiers AA4 and AA5 through a iterminal tA4 and resistors RA7 and RAlO, while a neg ative bias voltage is applied to the operational amplifiers AA7 and AA8 through another terminal tA5 and resistors RA and RA18. The diodes D, to D,, select the maximum positive voltage among the outputs ofthc operational amplifiers AA3 to AAS, and the diodes D to D select the maximum negative voltage among the outputs of the operational amplifiers AA6 to AA8. These positive and negative voltage outputs are applied to the adder composed of the resistors RA21 to RA23 and the operational amplifier AA9. This adder has a gain of unity and carries out simpleaddition. The output of the operational amplifier AA9 is changed in polarity by the sign change composed of the resistors RA24, RAZS and the operational amplifier AAlO. Of course, the gain of this sign changer is also selected to be unity. When now the output of the adder AD is negative, the outputs of the operational amplifiers AA6 to AA8 are positive and the diodes D, to D are cut off. Conversely, when the output of the adder AD is positive. the outputs of the operational amplifiers AA3 to AAS are negative and the diodes D, to D are cut off. Further, when the output of the adder AD islow and has a small positive or negative value, the outputs of the operational amplifiers AA4, AAS and AA7, AA8 are rendered negative and positive respectively by the effect of the bias voltages and the diodes D D;,, D,-, and D are cut off. When, for example, the output of the adder AD is initially negative and is then increased to such a value which will cancel the effect of the bias voltage, the output of the operational amplifier AA4 becomes positive. With the further increase in the output of the adder AD. the output of the operational am- .plifier AA4 will finally exceed the output of the operational amplifier AA3. In such a case. the output of the operational amplifier AA4 passes through the operational amplifiers AA9 and AAlO to appear as AP.

Therefore. the output AP of the frequency control circuit AF in FIG. 4 varies relative to AF as shown in FIG. 6. It will be seen from FIG. 6 that, when AF is positive, that is, when the output ofthe adder AD is negative, the outputs of the operational amplifiers AA3 and AA4 appear as shown by the dotted lines and the maximum value of these outputs appears as AP as shown by the solid line. Similarly, when AF is negative, the outputs of the operational amplifiers AA6 to AA8 appear as shown by the dotted lines and AP asshown by the solid line can be obtained. The inclination of the lines representing the outputs of these operational amplifiers AA3 to AA8 is determined by the ratio between the input resistor and the feedback resistor as described previously. The points P, to P, are determined by the magnitude of the bias voltages applied to the terminals tA4 and tAS and by the ratio between the input resistor and the feedback resistor relati e to the bias voltage. Therefore. in the embodiment shown in'FlG. 4, the greater the frequency difference, G, in the equation (2) becomes greater, and the frequency difference remaining after control can be correspondingly reduced.

However, when G, in the equation (2) is greater than a certain limit, the loop gain may be increased and the control system may be rendered unstable. The embodiment shown in FIG. 4 is inadequate in such a case.

Another embodiment shown in FIG. 7 can be advantageously employed to deal with such a case. FIG. 7 il lustrates solely a frequency control circuit AF which is adapted to operate always with the same gain so as to reduce an undesirably large frequency difference re maining after control. I

, ence becomessmall.

- V Referring to FIG. 7, the frequency control circuit AF comprises-operational amplifiers AAll to AA16, resistors RA31 to RA46, diodes D11 to D14, and bias terminals tA6 to tA9. Negative bias voltages of different levels are applied to the bias terminals tA6 and tA7, and positive bias voltages of different levels are applied to the bias terminals tA8 and tA9. Suppose that the relation between AF and AP is similar to that described with reference to FIG. 4, thena negative output appears from the adder. AD when the frequency of the system B is reduced. Therefore, by suitably determining the ratio between the input resistor RA41 and feedback resistor RA46, an output having a characteristic as shown the dotted line in FIG. 8 appears from the op erational amplifier AA16. The output of the adder AD is subjected to sign change by the operational amplifier AAll having a gain of unity determined by the input resistor RA31 and feedback resistor RA32, and the output of the operational amplifier MM is applied to the operational amplifiers AA12 to AAlS. Due to the fact that the bias voltages of the illustrated polarities are applied to these operational amplifiers AA12 to AAlS, all the diodes D11 to D14 are in the conducting state and the outputs of the operational amplifiers AA12 to AAlS are Zero unless the polarity of the output of the operational amplifier AAll is changed and the absolute value thereof exceeds those of the bias voltages. Suppose now that AF is increased to Af, shown in FIG. 8 and the absolute value of the output of the operational amplifier AAll is equal to that of the bias voltage applied to the bias terminal tA6. In this case, the composite input to the operational amplifier AA12 is positive and the diode D11 is cut off so that a negative constant voltage appears from the operational amplifier AA12. Therefore, the output of the operational amplifier AA16 is increased by AP, which is determined by the ratio between the input resistor RA42 and feedback resistor RA46 as shown in FIG. 8. The bias voltage applied to the bias terminal tA7 may be selected so that the diode D12 is cut off when AF Af Thus, when AF is increased to satisfy the relation AF Af. the output of the operational amplifier AA16 is increased by AP- which is determined. of course, by the ratio between the input resistor RA43 and feedback resistor RA46. The bias voltages applied to the bias terminals tA8 and tA9 may be selected so that the diodes Dl3and D14 are cut off when AF changes in the negative direction to satisfy the respective relations AF Af and AF Af,. In this manner, the characteristic shown in FIG. 8 can be completely obtained. Constant positive voltages delivered from the operational amplifiers'AAl4 and AA15 are applied to the operational amplifier AA16 through the input resistors RA44 and RA45. The positive and negative characteristics shown in FIG. 8 can be rendered entirely the same when the resistors RA42 to RA45 are selected to satisfy the relations RA42 RA44 and RA43 RA45.

The embodiments shown in FIGS. 4 and 7 are effective in reducing the frequency difference remaining after control. In these embodiments, however, a frequency difference of some extent may still remain due tothe fact that G, in the equation (2) takes the original small value again when the resulting frequency differ- Another embodiment of the present invention for furth'er reducing'the residual frequency difference will be "ciescribecl with reference to FIG. 9. The embodiment shown in FIG. 9 is basically similar to the embodi ment shownin FIG. 7 but differs from the latter in that there is a hysteresis in the AF AP characteristic in both the cases in which the frequency difference is increased and decreased as shown in FIG. 10. More precisely, as seen in FIG. 10, AP is increased by AP,'when AF is increased to satisfy the relation AF Af,, but even' when AF is decreased to satisfy'the relation AF Af,, AP, is not immediately reduced to zero and is reduced to zero after AF is decreased to satisfy the rela tion AF 3f,'. Similarly,-AP is increased by AP when AF is increased to satisfy the relation AF Af and AP is removed when AF is decreased to satisfy the relation AF Af The sameapplies to the case in which AF changes in the negative direction, and'AP, and AP are removed when AF is restored to respective levels Afg and Af., relative to Af,-, and Af as will be readily seen from FIG. 10.

In FIG. 9, like reference numerals are used to denote like parts appearing in FIG. 7 and additional parts having functions similar to those shown in FIG. 7 are designated by dashed reference numerals. Further, inorder to avoid confusion, circuits operating under the conditions AF Af and AF Af are shown merely by dotted blocks with Afg and Af, affixed thereto. Referring to FIG. 9, operational amplifiers AA17 to AA and associated resistors RA47 to RA54 act as sign changers whose gain is unity. Flip-flops FFl and FF3 are set when the outputs of respective operational amplifiers AA12 and AA19 change from zero to a constant negative voltage and are reset when the outputs of respective operational amplifiers AA12 and AA20 change from a constant negative voltage to zero. In the set po sition, these flip-flops FF 1 and FF3 deliver a constant negative voltage. Therefore, when now bias voltage of illustrated polarities corresponding to Af, and Afare applied to terminals tA6' and tA8 respectively, it will be easily understood from the description given with reference to FIG. 7 that the flip-flop FF 1 is set when AF Af, and is reset when AF Af,'. In response to the setting of the flip-flop FFl, a stepwise changing output appears from the operational amplifier AAI6 as in the embodiment shown in FIG. 7. It will be also easily understood that the flip-flop F F3 is set and reset when AF Afi, and AF Af respectively. It is apparent that the characteristic shown in FIG. 10 can be realized when the blocks labelled Af and Af whose detailed structure is not illustrated have a structure similar to that of the flip-flops FFl and FF3..

In FIG. 10, the AF AP characteristic draws hysteresis loops depending on the value of AF Another embodiment of the present invention shown in FIG. 11 differs from the embodiment shown in FIG. 9 in that a AF AP characteristic as shown in FIG. 12 is obtained. More precisely, as seen in FIG. 12, the AF AP characteristic is shifted parallelly as shown by GA, OB, O"C and GA, O'B', O"C'when detection of AF Af, and AF Af lasts for more than predetermined periods of time t, and respectively, and this characteristic is restored toward OA and 0A when the states of AF Af, and AF Af last for more than predetermined periods of time I, and 1 respectively. In FIG. 11, like reference numerals are used to denote like parts appearing in FIG. 9. Timers T, and T generate an output when operational amplifiers AA12 and AA19 deliver a constant negative output, that is, in the predetermined periods of time I, and t after detection of AF Af, and

AF Af respectively. Timers T, and T generate an output when operational amplifiers AA12 and AA20 delivering a constant negative output case to deliver such output, that is, in the predetermined periods of time 1, and 1 after detection of AF Af, and AF Af, respectively. Flip-flops FFl to FF4 are set by the outputs of the timers T, and T and are reset by the outputs of the timers T, andT- respectively. The output of an operational amplifier AAI6 changes stepwise as in FIG. 9'depending on the outputs of these flip-flops FF 1 to FF4.

Therefore, in the embodiment shown in FIG. 11, a constant negative output appears from the operational amplifier AA12 to energize the timers T, and T when AF is increased to give the relation AF Af,. When this state continues for more than the predetermined period of time t,, an output appears from the timer T, and the output of the operational amplifier AAI6 makes a stepping change to'OB. WhaLthe operational amplifier AA12 continues to deliver still its output in spite of the change of the output of the operational amplifier AAI6 to the level O'B, an output appears from the timer T in the predetermined period of time (t, 1 Therefore, the output of the operational amplifier AAI6 makes a stepping change from 0'B to O"Cl As a result of the increase in AP for increasing the power being transmitted AF is decreased until finally the relation AF Af, is satisfied. When this relation is satisfied, the constant negative output of the operational amplifier AA12 ceases to appear with the result that the timers T, and T are energized. The AF AP characteristic is restored from 0"C to 0'8 when this state continues for rriore than the predetermined period of time 1 and after another predetermined period of time t, (t. t,), the AF AP characteristic isrestored to 0A. Similar operation is carried out when AF changes in the negative direction. It will thus be understood that, in the embodiment shown in FIG. 11, the basic control operation takes place after substantially complete elimination of the frequency difference so that the frequency difference which may still remain after control can be reduced to a minimum. A very effective frequency control can be attained especially when the set tings of the timers in FIG. 11 are determined in relation to the operating timing of the governors for the generators in the individual a.c. systems.

It will be understood from the foregoing detailed description of the present invention that the difference between the frequencies of two a.c. systems interconnected by a dc. system can be reduced to a minimum by the effect of do. interconnection even when failure of proper power supply occurs in one of the interconnected a.c. systems resulting in a great reduction of the frequency. The present invention is especially effective in stabilizing the f equency of an a.c. system the scale of which is small compared with that of another a.c. system in an interconnected arrangement of two a.c. systems.

The embodiments described hereinbefore have referred to the case in which AP relative to the frequency difference AF is controlled over three stages depending on the values of AF, but it is apparent that a more precise control can be attained when the number of these stages is further increased.

Further, the foregoing description has referred to the manner of control for reducing the frequency difference AF between two a.c. systems. However, when the capacity of one of the two a.c. systems is smaller than the other and it is desired to stabilize the frequency .of the a.c. system of smaller capacity, the mannerv of control may be such that the error AF relative to the standard frequency of the specific a.c. system may be reduced to a minimum,

Further, when the two a.c. systems interconnected by the dc. system have different standard frequencies, AF should be normalized frequency difference, or percent difference in relation to the respective standard fre quencies.

From the standpoint of merely eliminating an undesirable offset remaining after control, the frequency control circuit AF shown in FIG. 1 may be modified into a so-called proportional plus integral type of control circuit so that the power in the dc. transmission line can be increased by the integrating means until the offset is eliminated. However, resorting to such integrating means for the elimination of the offset is disadvantageous. More precisely, the method in which the circuit for integrating the frequency difference AF is provided in parallel with the feedback route for the frequency difference AF is disadvantageous in that the gain thereof cannot be appreciably increased from the viewpoint of the stability of control and the purpose of reducing the error within a short period of time cannot be attained. Therefore, additional disposition of such integrating circuit in the present invention is effective only when a higher precision is demanded at the sacrifice of the period of time required for control, but the method resorting to this integrating circuit for the elimination of the offset is undesirable.

Another important advantage of the present invention over the system employing the integrating circuit is that stability of a.c. systems can be more reliably maintained than in the latter. It is difficult to maintain stable operation of an a.c. system when the frequency thereof is lowered to a value below a predetermined limit. Therefore, supply of power from another a.c. system becomes especially important with the increase in the degree of lowering of the frequency. According to the present invention, the effective gain is increased with the increase in the degree of lowering of the frequency and the reinforcing power can be increased within a short period of time.

It is apparent that the present invention is in no way limited to specific embodiments above described and various modifications may be made in the circuitry thereof. Further, it is apparent that, even when the control system shown in FIG. 4 does not include so-called constant power control means .and includes merely constant current control.means and constant margin angle control means, the present invention may be applied directly to such a system by utilizing AP for modifying the set point of the constant current control means.

What is claimed-is:

l. A control systemfora system interconnection consisting of two a.c. systems interconnected by a dc. power transmission system,-comprising a rectifier and an inverter in said dc. power transmission system, means including at least constant current control means associated with said rectifier for controlling the current in said dc power transmission system to a constant value, and means for modifying the set point of said constant current control means depending on the frequency difference between the two a.c. systems so that the set point of said constant current control means can be greatly modified with the increase in the frequency difference, said means for modifying including first means for detecting the frequencies of the respective a.c. systems to convert the same into corresponding voltage signals, second means for seeking the difference between the frequencies of the two a.c. systems on the basis of the voltage signal outputs of said first means, third means for amplifying the output of said third means representing the frequency difference, and fourth means connected to said third means for delivering a signal for-modifying the current setting of the d.c. power transmission system, said third means having a non-linear characteristic so that the rate of increase or decrease in the power being tramsmitted through the dc. power transmission system can be greatly increased with the increase in the frequency difference.

2. A control system as claimed in claim 1, comprising a plurality of comparators for comparing the frequency difference with a plurality of different settings, and an adder for seeking the sum of the frequency difference and the outputs of said comparators, the output of said adder providing said modifying signal.

3. A control system as claimed in claim 2, wherein said comparators are set to have a hysteresis characteristic in providing the outputs relative to the frequency difference.

4. A control system claimed in claim 3, wherein said hysteresis characteristic is obtained by provision of timer means in said control system. 

1. A control system for a system interconnection consisting of two a.c. systems interconnected by a d.c. power transmission system, comprising a rectifier and an inverter in said d.c. power transmission system, means including at least constant current control means associated with said rectifier for controlling the current in said d.c. power transmission system to a constant value, and means for modifying the set point of said constant current control means depending on the frequency difference between the two a.c. systems so that the set point of said constant current control means can be greatly modified with the increase in the frequency difference, said means for modifying including first means for detecting the frequencies of the respective a.c. systems to convert the same into corresponding voltage signals, second means for seeking the difference between the frequencies of the two a.c. systems on the basis of the voltage signal outputs of said first means, third means for amplifying the output of said third means representing the frequency difference, and fourth means connected to said third means for delivering a signal for modifying the current setting of the d.c. power transmission system, said third means having a non-linear characteristic so that the rate of increase or decrease in the power being tramsmitted through the d.c. power transmission system can be greatly increased with the increase in the frequency difference.
 2. A control system as claimed in claim 1, comprising a plurality of comparators for comparing the frequency difference with a plurality of different settings, and an adder for seeking the sum of the frequency difference and the outputs of said comparators, the output of said adder providing said modifying signal.
 3. A control system as claimed in claim 2, wherein said comparators are set to have a hysteresis characteristic in providing the outputs relative to the frequency difference.
 4. A control system as claimed in claim 3, wherein said hysteresis characteristic is obtained by provision of timer means in said control system. 